1. Field of the Invention
This invention relates to the stabilisation of solutions of permonosulphuric acid.
2. Brief Description of the Prior Art
Solutions of permonosulphuric acid may be produced by reacting sulphuric acid and hydrogen peroxide in the presence of water. In order to prevent undue decomposition of the hydrogen peroxide and a reduction in the available oxygen content of the solution the reaction mixture is preferably formed by introducing an aqueous solution of hydrogen peroxide into a quantity of concentrated sulphuric acid while controlling the temperature, if necessary by cooling, to avoid the evolution of nascent oxygen, at a temperature preferably below 20.degree. C. particularly preferably below 15.degree. C.
Hydrogen peroxide is not normally commercially available at concentrations substantially above about 85% by weight in water, so-called "high test" hydrogen peroxide. While high test hydrogen peroxide may very suitably be utilised to form the reaction mixture this is not essential. Preferably the hydrogen peroxide, as introduced, has a concentration of at least 50%, and particularly preferably of at least 60% by weight.
The concentration of sulphuric acid in the solution is an important feature in that, if the said concentration is below about 60% by weight only a low concentration of permonosulphuric acid can be achieved or maintained. The presence of higher concentrations of sulphuric acid in the stripping solutions enables higher concentrations of permonosulphuric acid to be achieved. In a test an aqueous solution made up by the addition of 85% hydrogen peroxide to 75 % weight of H.sub.2 SO.sub.4 contained 1.43% weight of hydrogen peroxide and 4.22% wt of permonosulphuric acid. Further solutions were produced using the same recipe each modified only in that the initial concentration of the sulphuric acid was increased to 80%, 85%, 90% and 95% by weight. The % by weight of permonosulphuric acid and residual hydrogen peroxide in each of these solutions was determined experimentally and is set out in the following Table 1.
TABLE 1 ______________________________________ % wt H.sub.2 SO.sub.4 % wt H.sub.2 SO.sub.5 % wt Residual H.sub.2 O.sub.2 ______________________________________ 75 4.22 1.43 80 5.46 0.89 85 6.88 0.31 90 6.66 0.21 95 6.75 0.10 ______________________________________
Suitably the concentration of sulphuric acid in the stripping solutions is greater than 60% by weight, preferably greater than 75% by weight and particularly preferably greater than 85% by weight and the concentrations and ratios of the ingredients are controlled accordingly.
The quantity of hydrogen peroxide relative to that of sulphuric acid may suitably be such as to produce from 0.2% to 10%, particularly preferably from 2% to 10% by weight of permonosulphuric acid according to the equation EQU H.sub.2 SO.sub.4 +H.sub.2 O.sub.2.function.H.sub.2 SO.sub.5+H.sub.2 O
Solutions of permonosulphuric acid are of use in industry, for example as photoresist stripping solutions for use in the course of the production of integrated circuitry based on semi-conductor wafers in the electronics industry. There have been proposals, for example in Japanese Patent Publication No. 50-101107 and to produce such stripping solutions at the point of use. U.S. Pat. Specification No. 3900337 discloses the production of photoresist stripping solutions based on a mixture of sulphuric acid and hydrogen peroxide which have a storage life of 3 weeks at ambient temperature.
It would be advantageous to produce solutions of permonosulphuric acid having an extended storage life.
There have been proposals to produce long term storagestable solutions of permonosulphuric acid notably in European Patent Publication 0256284. To attain this objective that Publication teaches the observance of a procedure for the formation of the reaction mixture for the production of such solutions, the use of electronic grade reagents, the inclusion of a sequestrant and the inclusion of a stabilising quantity of a soluble tin compound, very suitably a stannate such as an alkali metal stannate or stannous oxide, or any other tin compound which forms a hydrous tin oxide in the reaction mixture.